Решение задачи №99 от UniLecs (непростая сортировка)

puzzling_sort.rs

use std::borrow::Borrow;
use std::cmp::Ordering;

fn num_part_compare(a: &[u8], b: &[u8]) -> Ordering {
    let a_b = a.iter().chain(b.iter());
    let b_a = b.iter().chain(a.iter());
    
    for (x, y) in a_b.zip(b_a) {
        match x.cmp(y) {
            Ordering::Equal => (),
            non_eq => return non_eq.reverse()
        }
    }
    
    Ordering::Equal
}

fn greatest_concatenated_mut<S>(arr: &mut [S]) -> String
where
    S: Borrow<str> + Ord,
{
    arr.sort_unstable_by(|a, b| num_part_compare(a.borrow().as_bytes(), b.borrow().as_bytes()));
    arr.concat()
}

fn greatest_concatenated<S>(arr: &[S]) -> String
where
    S: Borrow<str> + Ord
{
    let mut buf = arr.iter().map(Borrow::borrow).collect::<Vec<_>>();
    greatest_concatenated_mut(&mut buf)
}

#[test]
fn some_variants() {
    let arr = ["123", "124", "56", "90"];
    let answer = "9056124123";
    assert_eq!(&greatest_concatenated(&arr), answer);

    let arr = ["1", "100", "10"];
    let answer = "110100";
    assert_eq!(&greatest_concatenated(&arr), answer);

    let arr = ["5", "54", "56"];
    let answer = "56554";
    assert_eq!(&greatest_concatenated(&arr), answer);

    let arr = ["89", "30", "309", "300", "81", "8"];
    let answer = "8988130930300";
    assert_eq!(&greatest_concatenated(&arr), answer);
    
    let arr = ["53", "5354"];
    let answer = "535453";
    assert_eq!(&greatest_concatenated(&arr), answer);
    
    let arr = ["53", "5352"];
    let answer = "535352";
    assert_eq!(&greatest_concatenated(&arr), answer);
}

Playground: http://play.rust-lang.org/?gist=8f2716d596e362aad4fc251efdd6f227&version=stable&mode=debug